Systema Porifera: A Guide to the Classification of Sponges, Edited by John N.A. Hooper and Rob W.M. Van Soest
© Kluwer Academic/Plenum Publishers, New York, 2002
Class Hexactinellida Schmidt, 1870
Henry M. Reiswig
Natural History Section, Royal British Columbia Museum, 675 Belleville Street, Victoria, British Columbia, Canada V8W 9W2, and Biology
Department, University of Victoria, P.O. Box 3020 STN CSC, Victoria, British Columbia, Canada V8W 3N5. (hmreiswig@shaw.ca)
Hexactinellida Schmidt (Porifera) are deep-water marine sponges defined by their production of siliceous spicules of hexactinic, triaxonic
(cubic) symmetry, or shapes clearly derived from such forms by reduction of primary rays or terminal branches added to the ends of primary rays. They lack calcareous minerals and sclerified organic spongin as skeletal components. Siliceous spicules may be entirely loose,
or partially fused to form a rigid basal and choanosomal framework. Their living tissues are mainly syncytial, with distinctive porous
plugs joining differentiated regions of the syncytium to each other or to discrete cellular components. Flagellated-collar units are anucleate. Hexactinellids are viviparous and, from detailed study of a single species, produce distinctive trichimella larvae. Two subclasses are
recognized by different microsclere forms – amphidiscs and hexasters. Hexactinellids include about 500 described species, 7% of all
Porifera, distributed in 5 orders, 17 families and 118 genera.
Keywords: Porifera; Hexactinellida; Amphidiscophora; Hexasterophora; Amphidiscosida; Aulocalycoida; Hexactinosida; Lyssacinosida;
Lychniscosida.
DEFINITION, DIAGNOSIS, SCOPE
Synonymy
Hexactinellidae Schmidt, 1870. Coralliospongia (in part)
Gray, 1867a. Vitrea Thomson, 1868. Hyalospongiae (in part)
Claus, 1872. Sexradiatospongia Gray, 1874. Triaxonia Schulze,
1886.
Definition
Porifera with siliceous spicules of triaxonic (cubic) symmetry
or derivations of that basic form. Typical spicules are hexactine in
form, the three axes intersecting at right angles; loss of one or more
rays results in spicules of pentactine, tetractine, triactine, diactine
or monactine form. The axial filament is square in section.
Diagnosis
Body form variable, including monomeric tube-, cup-, funnel-,
cap-, club-, lobate- and blade-forms, with or without lateral diverticula, and branching tubule or solid cylinders, with or without
anastomoses; encrusting forms unknown. Entirely marine on hard
or soft substrate; attached directly to hard substrate either firmly
by basal disc cementation (basiphytous) or loosely by grappling
anchor spicules (lophophytous); attached to soft substrate by rooting spicules (lophophytous) or rarely by solid roots (rhizophytous).
Skeletons of (mineral) siliceous spicules, as separate elements
or joined by silica deposition and (organic) thin lattice of collagen;
dense spongin, calcareous deposition and aspicular forms
unknown. Siliceous spicules generally divisible into megascleres
and microscleres on basis of form, size, and function; intermediatesize surficial spicules sometimes distinguished as mesoscleres.
Megascleres basically hexactins but reduction of one or more rays
results in pentactins, tetractins (usually stauractins), triactins
(usually tauactins), diactins and monactins (usually basal anchors,
sceptrules, or sceptres); megasclere ray branching is rare. Rigid
skeletal frames of hexactine megascleres often fused at or soon
after spicule formation along parallel rays or at ray crossing points
are known as dictyonal frameworks; rigid frames formed by fusion
of non-hexactine megascleres (usually diactins) long after spicule
formation in more basal body regions of sponges with otherwise
separate spicules are not considered dictyonal frameworks. Living
tissues are mainly syncytial, including dermal and atrial membranes, internal trabeculae and flagellated chamber walls with
anucleate collar bodies. Discrete nucleate cellular components
embedded in pockets or capsules of the syncytium may be joined
together or to syncytium by distinctive porous plugs or may be
entirely separate. Cellular differentiation is slight. Translocation
of intrasyncytial materials is by symplastic flow transport across
plugs and cytoplasmic streaming within open syncytia; stimuli are
disseminated throughout entire individuals by membrane conduction. The flagellated chambers are large and eurypylous, arranged
between very thin-walled inhalant and exhalant water conducting
canals in syconoid, sylleibid or leuconoid pattern. All known members are active water pumpers and particle filterers; food particle
acquisition by passive flow and sedimentation appears minor or
non-existent. Larval form, the trichimella, is well-known for only
one species; among many distinctive features it has an equatorial,
subepidermal sheath of multiflagellate cells, the flagella of which
project through pores of the overlying syncytial epidermis to
provide motility. Reproductive strategy is viviparous in the few
forms where known.
Remarks
Hexactinellida presently contains about 500 Recent species or
7% of all described Recent Porifera. They have been reported from
depths of 5–6770 m and are unknown in freshwater or intertidal
habitats. They constitute important members of deep-water marine
communities, many of which remain unsampled or poorly sampled, and in those regions that have supposedly been well sampled,
new species continue to be discovered. It is likely that the total
number of species will exceed 1000 when present institution
collections are reviewed and unsampled areas are surveyed.
In early taxonomic treatments (e.g., Schulze, 1886), the class was
divided on presence or absence of a fused dictyonal framework –
present in Dictyonina and absent in Lyssacina. Realization that
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Porifera • Hexactinellida
accessory spicules, mainly microscleres, were shared by some
members of both lyssacine and dictyonine groups led Schulze
(1899) to create a new primary division based upon microsclere
form: Amphidiscophora bearing amphidiscs and Hexasterophora
bearing hexasters. These remain the two well-differentiated
subclasses of Hexactinellida today despite recent discovery
(Tabachnick in Tabachnick & Lévi, 1997b) of amphidiscs among
hexasters in some Hexasterophora and hexadiscs (not hexasters)
among typical amphidiscs in some typical Amphidiscophora. This
shared similarity is believed to be a recent convergent feature and
the two subclasses are still considered to have a long independent
evolution since the Paleozoic (Mostler & Mehl, 1990). Tests of
the distinction between the two subclasses by genetic sequence
analysis have not yet been carried out but may be expected shortly.
Such analyses using Hexasterophora sequences (Collins, 1998;
Kruse et al., 1998; Adams et al., 1999; Borchiellini et al., 2001;
Medina et al., 2001) have not yet generated consistent stronglysupported relationships among Porifera classes, and are unlikely
to influence systematic organization at high taxonomic levels for
some time.
Scope
In its Recent composition, the class contains two subclasses,
five orders with 17 families and 118 valid genera, 111 of which are
placed to family and seven of which remain incertae sedis in two
orders. Recent orders are: (1) Amphidiscophora: Amphidiscosida;
(2) Hexasterophora: Aulocalycoida, Hexactinosida, Lychniscosida,
Lyssacinosida.
Recent reviews
Reid, 1958a; Hartman, 1982; Mehl, 1992; Hooper &
Wiedenmayer, 1994.
tauactins, diactins or some combination of these; hypodermalia
usually as pentactins, rarely hexactins and stauractins.
Remarks. The subclass presently contains a single Recent
order with three families.
Subclass Hexasterophora Schulze, 1886
(emend. Schulze, 1899)
Other names. None.
Definition. Basiphytous, rhizophytous or lophophytous
Hexactinellida in which microscleres are mainly or all astral, typically hexasters; amphidiscs occur rarely as variants of discohexactins; choanosomal skeletons may consist of entirely loose
spicules, sometimes irregularly joined by synapticulae or silica
cement at contact points, or of regularly fused hexactins forming
a dictyonal framework; dictyonal nodes, where present, may be
entirely simple or lychniscid (lantern nodes).
Remarks. Hexasterophora was first erected by Schulze
(1886) as a tribe of the suborder Lyssacina Zittel, containing only the
hexaster-bearing lyssacine hexactinellids; he assigned the hexasterbearing dictyonine hexactinellids to a separate suborder, Dictyonina
Zittel. After considering a reorganization of the entire Hexactinellida
(Schulze, 1893), Schulze (1899) finally abandoned Zittel’s (1877)
primary division of hexactinellids on the basis of lyssacine vs.
dictyonine parenchymal skeletons, citing the presence of dictyonal
tendencies at the attachment of some lyssacine Hexasterophora
(basidictyonalia in Rhabdocalyptus mirabilis) and the existence of
some hexaster-bearing genera that could be regarded as intermediates between lyssacine and dictyonine forms (e.g., Euryplegma) for
his action. He joined all hexaster-bearing lyssacine and dictyonine
hexactinellids into one of two major subdivisions (hierarchy level
unspecified) of Hexactinellida as his emended Hexasterophora, and
retaining his original tribe Amphidiscophora as the other major
subdivision. His early insight has stood the test of time and remains
the basis for subclass definitions.
SUBCLASSES OF HEXACTINELLIDA
Conclusions
Subclass Amphidiscophora Schulze, 1886
Present subclass division of Hexactinellida has been and
remains stable and non-controversial. Should evidence be discovered
that amphidisc and hexaster microscleres are not independently
evolved characters (from new fossil developmental information
sources), or that some members of Hexactinosida share a more
recent common ancestry with Amphidiscophora than with other
Hexactinosida (e.g., through genetic sequence analysis), the relationships between the two subclasses would need reconsideration.
Other names. None.
Definition. Lophophytous Hexactinellida which always
contain amphidisc microsleres or some variant of them, and triaxon
microscleres which never bear terminal ray branching (never astral
microscleres); choanosomal skeleton of loose megascleres, never
fused together, in the form of hexactins, pentactins, stauractins,
KEY TO ORDERS OF RECENT HEXACTINELLIDA
(1) Amphidisc microscleres present; hexaster microscleres absent ................................................................................. Amphidiscosida
Astral (hexasters) microscleres present; amphidisc microscleres rare .............................................................................................. (2)
(2) Dictyonal framework formed of hexactins fused by secondary silicification ................................................................................... (3)
Dictyonal framework of fused hexactins absent; fusion of non-hexactine megascleres (diactins, triactins, tetractins) may occur in
older parts ....................................................................................................................................................................... Lyssacinosida
(3) Dictyonal nodes mainly lychniscid, but some may be simple ....................................................................................... Lychniscosida
Dictyonal nodes simple ..................................................................................................................................................................... (4)
(4) Dictyonal meshes consistent in size and shape; dictyonal rays one mesh in length; dictyonal strands as serially aligned
beam pairs ...................................................................................................................................................................... Hexactinosida
Dictyonal rays irregular in size and shape; dictyonal rays exceed mesh length; dictyonal strands as long single rays or multiple (2)
overlapping rays ............................................................................................................................................................. Aulocalycoida